The present invention relates to a natural draft cooling tower with a plurality of heat exchanger elements, preferably in the shape of a pitched roof, for condensing turbine exhaust steam of a power plant, wherein the heat exchanger elements, supplied with the exhaust steam by a common central steam inlet line and radially extending distributing lines, operate in part in a condensational and in part in a dephlegmational manner, with the dephlegmationally operating heat exchanger elements arranged downstream of the condensationally operating heat exchanger elements, the heat exchanger elements distributed over a plurality of identical sectors, each sector having complete lines for steam distribution as well as inert gas and condensate removal lines.
Such natural draft cooling towers for condensation of turbine exhaust steam of a power plant are known from German Offenlegungsschrift 34 41 514. Since the collection of inert gases within the heat exchanger elements must be prevented, the final condensation is performed in the dephlegmationally operating, forced-vented heat exchanger elements from which the inert gases are removed. In order to provide the dephlegmationally operating heat exchanger elements with a sufficient& amount of cooling air during all conceivable load situations and during unfavorable weather conditions, these dephlegmationally operating heat exchanger elements are provided with their own fans. These fans ensure a complete final condensation of the entire turbine exhaust steam to be condensed within the dephlegmationally operating heat exchanger elements even under unfavorable weather conditions such as strong lateral winds and inversion situations and provide also the option that, due to the resulting additional draft power within the cooling tower that is otherwise operated only by natural draft, the cooling tower shell can be designed with a respectively reduced height to thereby lower construction costs.
From German Auslegeschrift 19 60 619 a symmetrically designed natural draft cooling tower is known in which the heat exchanger elements which are essentially in the shape of a pitched roof are arranged radially with respect to the Iongitudinal axis of the cooling tower. The steam to be condensed is supplied via a centrally arranged steam inlet line from which radially extending steam distribution lines branch off to the ridge of the respective pitched roof-shaped heat exchanger elements. These operate in part in a condensational and in part in a dephlegmational manner whereby the dephlegmationally operating heat exchanger elements are arranged on an inner circle about the steam inlet line. One dephlegmationally operating heat exchange element is coordinated with two condensationally operating heat exchanger elements which are arranged in the radial extension of the dephlegmational heat exchanger element so that essentially an arrangement of heat exchanger elements within individual segments results. All of the heat exchange elements as well as their corresponding lines are arranged on a single common support construction which is supported on shoulders of the outer shell of the natural draft cooling tower.
From German Offenlegungsschrift 24 05 999 a natural draft cooling tower with a radial arrangement of the individual heat exchanger elements is known. The condensationally operating heat exchanger elements are arranged on an outer circle and the dephlegmationally operating heat exchanger elements are arranged on an inner circle close to the central steam inlet line. The arrangement of the condensationally operating heat exchange elements is two-stepped. A first condensational step is embodied by heat exchanger elements arranged adjacent to the outer wall of the cooling tower, and a second condensational step is provided by heat exchanger elements radially inwardly arranged and staggered relative to the heat exchanger elements of the first step. The second step is on a lower level than the first step. The heat exchanger elements of the second condensational step thus take over the remaining steam of two neighboring heat exchanger elements of the first condensational step. It is disadvantageous in this embodiment that due to the connection of the condensational heat exchanger elements with the respective neighboring radially displaced heat exchanger elements a reduced operation of the device with only a portion of the total number of heat exchanger elements present is not possible.
In a natural draft cooling tower known from German Offenlegungsschrift 22 42 058 condensationally operating heat exchanger elements are arranged on a plurality of circles about the central longitudinal axis of the cooling tower. The steam to be condensed is fed via steam inlet lines circularly arranged about the central longitudinal axis of the cooling tower. All heat exchanger elements of one circular arrangement are positioned on a common support construction in order to provide a staircase-like arrangement in the radially outwardly oriented direction by selecting respective suitable heights for the various circular arrangements.
Based on the disclosure of German Auslegeschrift 19 60 619 it is therefore an object of the present invention to provide a natural draft cooling tower which provides for a favorable adaptation of the respective condensation power to different operational conditions and/or to changing weather conditions and which, at the same time, provides for an optimal use of the base surface area of the cooling tower.